The invention relates generally to the control of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x) in the exhaust of internal combustion engines. More particularly, the invention relates to the removal of NO.sub.x when the exhaust gases include excess oxygen. This is typically the case with lean-burn engines, diesel engines, and other engines currently under development which are designed and produced to operate with amounts of oxygen beyond that needed for combustion of the fuel.
In recent years three-way catalysts have been used to remove all of the three principal noxious components in auto exhaust gases. The engines are run with stoichiometric air/fuel ratios and the catalysts are able to remove all three components at the same time, that is, a single catalyst is sufficient over the range of engine operating temperatures. More recently, development of so-called "lean-burn" engines is being driven by the desire to improve fuel economy. Such engines operate with air-fuel ratios which are far from the typical stoichiometric conditions. Instead of an air-fuel ratio of about 14.55/1 by weight, the lean-burn engine may operate with air-fuel ratios above 18/1, up to about 22-24/1, or even higher ratios for diesel engines. Under such conditions the engine exhaust may include less carbon monoxide, but will still contain excessive amounts of hydrocarbon and nitrogen oxides. Most catalysts are quite capable of converting hydrocarbons and carbon monoxide at such conditions, since the oxygen content is high, usually about 3-10% by volume. However, such conditions are not generally favorable for the reduction of nitrogen oxides. Much effort has gone into a search for catalysts that can effectively destroy nitrogen oxides under oxidizing conditions, but the results have not been satisfactory to date. Published information most pertinent to the present invention will be discussed below.
The use of base metals, particularly copper, ion-exchanged onto a zeolite support has been proposed by many workers in the art to be effective for reduction of nitrogen oxides. Others have suggested that such a catalyst should be combined with oxidation or three-way catalysts, usually in sequence. Toyota has proposed such arrangements in applications published in Japan. In JP Kokai 310742/1989 reference is made to earlier applications in which zeolites carrying transition metals are combined with downstream three-way or oxidation catalysts. These were said to be deficient and a catalyst was proposed which added noble metals to a copper-zeolite catalyst. Various methods of combining these materials are suggested. A related application is JP Kokai 127044/1989 in which an oxidation catalyst is deposited as a first layer, followed by a second layer of copper on a zeolite.
In EP 0488250A1 Toyota proposed three catalysts in series. Toyota discusses the use of various catalysts for removal of NO.sub.x from the exhaust of lean burn engines. Pt on zeolite is useful at low temperatures and Cu on zeolite at higher temperatures, but other catalysts are shown to be useful at inlet temperatures of about 300.degree. C. These are the noble metals combined with the oxides of rare earth metals and metals from Group IVa of the Periodic Table (IUPAC), such as Ti, Zr, and Hf.
In EP 0494388A1 the applicants disclose two stages of catalysts for first removing nitrogen oxides and then oxidizing the remaining hydrocarbons and carbon monoxide. The NO.sub.x removal catalysts are defined as phosphates, sulfates, or aluminates of transition metals of the 4th period of the periodic table (e.g. Cr, Mn, Fe, Co, Ni, Cu, Zn). The oxidation catalyst is generally described as a noble metal, a base metal or a perovskite on a support.
There are two generally recognized routes to removing nitrogen oxides. First, the nitrogen oxides can be completely decomposed to produce diatomic nitrogen and diatomic oxygen. This reaction is thermodynamically favored, but is extremely slow. Moreover, catalysts which are able to promote this reaction under the highly oxidizing conditions and high temperatures found in typical automotive engine exhaust have not yet been found. The second route is the chemical reduction of nitrogen oxides using as reducing agents those gases already present in the exhaust, such as carbon monoxide, hydrocarbons, and hydrogen. This is considered to be the mechanism of the three-way catalyst. However, such catalysts were originally developed to interact with the exhaust from an engine operating at or about a stoichiometric air-fuel ratio, thus containing little or no excess oxygen. When a large excess of oxygen is present, as in a lean-burn or diesel engine, the oxygen tends to preferentially react with the hydrocarbons, carbon monoxide, and hydrogen, thus removing those reducing agents usually needed to remove nitrogen oxides. The present inventors have found that this problem can be overcome.
Those working in this field intend to either decompose nitrogen oxides into the elements directly or to reduce them using reducing agents under oxidizing conditions. The catalysts and catalyst systems of the present invention are considered to function by reducing the nitrogen oxides rather than decomposing them. However, that conclusion was reached based on experimental evidence and is not an essential aspect of this invention. The reaction mechanisms by which nitrogen oxides are reduced are believed to vary depending on the catalyst and the operating temperatures.
This invention is based, at least in part, on the discovery that catalysts which function to promote exhaust gas purification under appropriate operating conditions of a lean burn vehicle engine will behave in a limited temperature range within those operating conditions to selectively convert nitrogen oxides. This was surprising, as it is generally contrary to the experience with three-way catalysts--which become active ("light-off") at atemperature of about 250.degree. to 350.degree. C. and thereafter are able to oxidize hydrocarbons and carbon monoxide while reducing nitrogen oxides over the full range of operating temperatures, say about 300.degree. to 800.degree. C.
It should be noted here that the composition of auto exhaust and its temperature changes as driving conditions change. Consequently, tests of auto exhaust catalysts require that a car be operated over a range of conditions representing typical driving. Once three-way catalysts have reached operating temperature, the performance is not greatly affected by the exhaust gas temperature. This is not the case, however, with catalysts for engines operating in the lean-burn mode. It has been found that such catalysts are effective in removing nitrogen oxides only over a limited temperature range and outside of such range they are not effective. As will be appreciated, such a characteristic is not compatible with the usual variation in engine exhaust temperatures that occurs during typical driving conditions. We have found that by proper selection of catalysts it is possible to remove nitrogen oxides from exhaust gases containing excess oxygen at temperatures within the full operating range of about 170.degree. C. to about 700.degree. C.
Selection of catalysts for this difficult task requires consideration of the characteristics of each catalyst. It is believed that the chemical reactions differ with the catalyst composition and the temperatures of operation. Consequently, it has not been possible to find a single catalytic component capable of covering the full temperature range of lean-bum engine exhaust in the same manner as has been done with engines operating with stoichiometric air-fuel ratios. Based on the results of our experiments we believe that there exist certain catalysts that are not only capable of reducing nitrogen oxides within a specific particular temperature range but still remain very effective for oxidizing hydrocarbons and carbon monoxide at other temperatures. Thus, it appears that providing for both the reduction of nitrogen oxides and the oxidation of their reducing agents involves not only selection of the proper catalysts but the proper combination and positioning of catalysts in order to achieve the desired reduction of nitrogen oxides throughout the range of operating conditions of a lean burn engine.